Temperature tracking voltage to current converter

ABSTRACT

A programmable and precise voltage-to-current converter (a.k.a. current source) that tracks temperature variations is presented. The voltage-to-current converter is implemented by placing a voltage reference circuit between the bases of the two transistors, or alternatively between a diode and a transistor, in a voltage controlled current source circuit which can be adjusted to track temperature variations. In one embodiment, the voltage reference circuit is a programmable digital-to-analog (D/A) converter. In a second embodiment, the voltage reference circuit is a differential amplifier.

FIELD OF THE INVENTION

The invention generally relates to signal converters, and moreparticularly relates to voltage-to-current converters.

BACKGROUND OF THE INVENTION

As part of their quality assurance, semiconductor device makerssystematically perform tests on their products to ensure that they meetor exceed all of their design parameters. Among the types of testsroutinely performed include device parametric testing (a.k.a. DCtesting), device logic function testing, and device timing testing(a.k.a. AC testing). While the semiconductor device being tested isoften referred to as the Device Under Test, the test system used inconducting the above tests on the DUT is often referred to as AutomaticTest Equipment (ATE).

The ATE is necessarily very precise to carry out the aforementionedtests on very sensitive DUT like semiconductor devices. In general, theATE hardware is controlled by a computer which executes a test programto present the correct voltages, currents, timings, and functionalstates to the DUT and monitor the response from the device for eachtest. The result of each test is then compared to pre-defined limits anda pass/fail decision is made. As such, the ATE hardware normally includea collection of power-supplies, meters, signal generators, patterngenerators, etc.

The Pin Electronics (PE) circuitry provides the interface between theATE and the DUT. More particularly, the PE circuitry supplies inputsignals to the DUT and receives output signals from the DUT. As anexample, in parametric testing, either an input voltage is sent to theDUT and an output current is received from the DUT or an input currentis sent to the DUT and an output voltage is received from the DUT.Accordingly, a programmable current source is one of the PE's requiredcomponents to drive desired currents to the DUT.

FIG. 1 illustrates, as an example, a prior art current source used in aPE circuitry. As shown in FIG. 1, prior art current source 100 comprisesdigital-to-analog (D/A) converter 101, bipolar transistors 102-103, andresistor R_(Iset). D/A converter 101 receives as inputs an analogreference voltage V_(refin) and a digital programmed value PV from thetest computer. In response, D/A converter 101 outputs an analog voltageV_(out). The output of D/A converter 101 is connected to resistorR_(iset) which in turn is connected to the collector of transistor 102.The base of transistor 102 is connected to the base of transistor 103.Moreover, the base of transistor 102 is also connected to the collectorof transistor 102. The emitter of transistor 102 is connected to a powervoltage V_(ref). While the emitter of transistor 103 is also connectedto voltage V_(ref), the collector of transistor 103 supplies the outputcurrent I_(out) of current source 100.

In so doing, transistors 102-103 and resistor R_(iset) form a currentmirror wherein a current is drawn away from the collector of transistor102 which causes an emitter-collector current to flow. Becausetransistors 102 and 103 are identical, a substantially equalemitter-collector current is provided as I_(out). Examining transistor102, from Kirchoff's voltage law:

    V.sub.EB +V.sub.BC +V.sub.CE =0

Because the base is connected to the collector, V_(BC) =0. As such, theabove equation becomes

    V.sub.CE =-V.sub.EB                                        (1)

From Ohm's law,

    I.sub.1 =(V.sub.ref -V.sub.BE -V.sub.out)/R.sub.iset       (2)

Well-known programmable D/A converter functional characteristics dictatethat

    V.sub.out =V.sub.ref *(PV/FS)                              (3)

where PV is the digital programmed value and FS is the full scaledigital value of the D/A converter.

Substituting equation (3) into equation (2),

    I.sub.1 =((V.sub.ref -V.sub.BE)-(V.sub.ref *(PV/FS)))/R.sub.iset =(V.sub.ref *(1-(PV/FS))-V.sub.BE)/R.sub.iset             (4)

From Kirchoff's current law,

    I.sub.Emitter +I.sub.Base +I.sub.Collector =0

Current I_(Base) is approximately equal to I_(Emitter) /H_(fe), whereH_(fe) is the transistor gain which is typically in the range of150-300. Therefore, I_(Base) is negligible compared to I_(Emitter) andI_(Collector). For this reason,

    -I.sub.Emitter =I.sub.Collector =I.sub.1                   (5)

Equation (5) is applicable to both transistors 102 and 103. BecauseI_(E) for both transistors 102 and 103 are the same,

    I.sub.1 =I.sub.out                                         (6)

Since it is well known that V_(BE) is related to temperature accordingto the equation:

    I.sub.E ˜exp(qV.sub.BE /kT)                          (7)

wherein q is the electronic charge, k is Boltzmann's constant, and T istemperature. Solving equation (7) for V_(BE),

    V.sub.BE ≈(kT/q)ln(I.sub.E)                        (8)

As can be seen from equation (4), I_(out) depends on V_(BE). Thus, underprior art current source 100, the output current I_(out) is affected bytemperature variations which in turn affect the precision of the currentsource. Moreover, prior art current source 100 error ΔV_(BE) @ΔT isconstant over the full operating range, as shown in FIG. 1A, making itimpossible to accurately program small values. This can be illustratedby the following example. Assume that V_(Ref) =5V, I₀ =1 mA, V_(BE)=0.6V, and that the D/A converter is a 12-bit converter. The resolutionfor this 12-bit D/A converter is 5V/2¹² bit=1.22 mV/bit. From equation(8), the change ΔV_(BE) with respect to temperature variations can bedetermined. However, for silicon as a material, it is common knowledgethat ΔV_(BE) =-2.5 mV/° C. Thus, a change of 1° C. represents a 200%error at the minimum current setting. Following this logic, a change of25° C.=-62.5 mV which translates to an error equal in magnitude to thelower 6 bits of a 12-bit D/A converter.

Referring now to FIG. 2 illustrating another prior art current source.As shown in FIG. 2, prior art current source 200 consists of adifferential amplifier whose output is connected to the bases of thetransistors in a current mirror circuit. The differential amplifierconsists of operational amplifier (op-amp) 201, resistor R_(I) 202,resistor R_(F) 203, resistor R_(I) 204, and resistor R_(F) 205.Resistors R_(I) 202 and R_(F) 203 are connected in parallel to thenon-inverted input of op-amp 201. Resistor R_(I) 202 is in turnconnected to reference voltage V_(Ref). Conversely, resistor R_(F) 203is in turn connected to ground. Resistor R_(I) 204 and R_(F) 205 areconnected in parallel to the inverted input of op-amp 201. ResistorR_(I) 204 is in turn connected to a voltage source V_(I). Resistor R_(F)205 is in turn connected to the output of op-amp 201.

The output of op-amp 201 is connected to resistor R_(set) 206 which inturn is connected to the collector of transistor 207 of the currentmirror. The bases of transistors 207 and 208 are connected together aswell as to the collector of transistor 207. The emitters of transistors207 and 208 are connected together as well as to voltage V₊. Finally,the collector of transistor 208 provides the output current for currentsource 200.

An circuit analysis of current source 200 shows that:

    I.sub.1 ≈I.sub.2 ≈I.sub.out I.sub.out =(V.sub.+ -V.sub.BE1 -V.sub.out)/R.sub.set                                     (9)

where V_(BE1) is the base-emitter voltage of transistor 207 and V_(out)is the output voltage of op-amp 201.

Since voltage V_(out) is also the output voltage of the differentialamplifier,

    V.sub.out =(V.sub.Ref -V.sub.i)*(R.sub.F /R.sub.I)         (10)

Substituting equation (10) into equation (9), the output current isdefined as:

    I.sub.out =(V.sub.+ -V.sub.BE -(V.sub.Ref -V.sub.i))*(R.sub.F /R.sub.I)/R.sub.set                                       (11)

where V_(BE) =V_(BE1) =V_(BE2).

Accordingly, like prior art current source 100, prior art current source200 depends on voltage V_(BE) which is subject to changes due totemperature variations which in turn greatly affect the precision of thecurrent source. As demonstrated earlier, a change of 1° C. represents a200% error at the minimum current setting. Moreover, prior art currentsource 200 error is constant over the full operating range making itimpossible to accurately program small values.

On the other hand, U.S. Pat. No. 4,251,743 issued Feb. 17, 1981 toHareyama (hereinafter Hareyama) discloses a current source designed forused in an Analog-to-Digital (A/D) converter which compensates fortemperature variations as well as changes of components' characteristicssuch as aging. The current source disclosed in Hareyama also implementsthe current mirror concept. However, the current source disclosed inHareyama implements feedback control (i.e., closed loop control) of itsoutput current I_(out) to compensate for errors. As a result, inaddition to requiring more hardware, the current source disclosed inHareyama requires may not be as precise and responsive as desired due tothe inherent characteristics (e.g., residual error and time lag) offeedback control.

Thus, a need exists for a precise current source circuit for use in acomputer controlled ATE which has good dynamic range that is able tocancel out or compensate for current changes caused by temperaturevariations.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a precise voltage-to-currentconverter (current source) circuit for use in a computer controlled ATEwhich has that is able to cancel out or compensate for current changesinduced by variations in the current source transistor's base-emittervoltage drop caused by temperature and process variations.

The present invention meets the above need with a current source circuitwhich comprises a voltage reference circuit and a voltage controlledcurrent source. The voltage controlled current source circuit has afirst and second transistor each having a base, a collector, and anemitter. In the preferred embodiment, the two transistors are co-locatedon a single substrate thereby insuring that they have similar electricaland thermal characteristics. The emitters of the first and secondtransistors are coupled to a first voltage. Moreover, the collector andthe base of the first transistor are connected together. The collectorof the second transistor provides an output current for the currentsource circuit.

The voltage reference circuit is coupled between the bases of the firstand second transistors. The voltage reference circuit can be adjustedeither manually or automatically to set or program the desired outputcurrent. The first transistor provides a temperature tracking referencefor the control element.

In one embodiment of the present invention, the voltage referencecircuit is a programmable digital-to-analog (D/A) converter. In anotherembodiment of the present invention, the voltage reference circuit is avariable differential amplifier.

All the features and advantages of the present invention will becomeapparent from the following detailed description of its preferredembodiment whose description should be taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a first prior art current source circuit.

FIG. 1A illustrates the I-V curves of the first prior art current sourcecircuit with the ΔV_(BE) @ΔT error band superimposed.

FIG. 2 illustrates a second prior art current source circuit.

FIG. 3 is a high-level block diagram illustrating a typical computercontrolled Automatic Test Equipment (ATE) that implements the presentinvention.

FIG. 4 is a block diagram illustrating a first embodiment of the currentsource circuit in accordance to the present invention.

FIG. 4A illustrates the I-V curves of the first embodiment of thecurrent source circuit in accordance to the present invention with theΔV_(BE) @ΔT error band superimposed.

FIG. 5 is a block diagram illustrating a second embodiment of thecurrent source circuit in accordance to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be obvious toone skilled in the art that the present invention may be practicedwithout these specific details. In other instances well known methods,procedures, components, and circuits have not been described in detailas not to unnecessarily obscure aspects of the present invention.Furthermore, while the following detailed description of the presentinvention describes its application primarily in Automatic TestEquipment (ATE), it is to be appreciated that the present invention canbe used in any apparatus or system requiring a current source.

FIG. 3 illustrates, for example, a high-level diagram of a computercontrolled ATE 300 in which the present invention may be implemented orpracticed. As shown in FIG. 3, ATE 300 may comprise computer system 301,system clocks and calibration circuits 302, formatting-masking-timesetmemory 303, pattern memory 304, system power supplies 305, specialtester options unit 306, precision measurement unit 307, DUT reference &power supplies 308, test head 309, and bin box 310.

Computer system 301 is the system controller. Computer system 301controls ATE 300 and supplies a means to transfer data to/from ATE 300.Hence, computer system 301 may generally include a central processingunit (CPU), input/output (I/O) interfaces such as parallel and serialports, communications interface for networking and communicating withthe outside world, video/graphics controller, a number of data storagedevices such as hard drive and tape drive for locally storinginformation, I/O devices such as keyboard and video monitor to allow theoperator to interact with ATE 300. It is to be appreciated that computersystem 300 can be any one of a number of different computer systemsincluding desk-top computer systems, general purpose computer systems,embedded computer systems, and others.

System clocks and calibration circuits 302 provide the ATE system clocksfor timing its operations and allow for ATE system calibrations. Patternmemory 304 is used to store test vector pattern data (i.e., arepresentation of the I/O states for the various logical functions thatthe DUT is designed to perform). Formatting-masking-timeset memory 303is used to store formatting, masking, and timeset data which modify testvector pattern data before sending it to the DUT as well as createsignal formats (wave shapes) and timing edge markers for input signalsand strobe timing for sampling output signals. System power supplies 305provide steady and uninterrupted alternating current (AC) power to ATE300.

Special tester options unit 306 contains optional circuits to allow ATE300 to be customized for carrying out predetermined tests. Precisionmeasurement unit 307 allows ATE 300 to make accurate direct current (DC)measurements. DUT reference & power supplies 308 supply DC power (e.g.,V_(DD), V_(CC), etc.) to the Device Under Test (DUT). Additionally, DUTreference & power supplies 308 supply input and output referencevoltages (e.g., VIL/VIH, VOL/VOH) to the DUT. Test head 309 contains pinelectronics (PU) circuitry and interfaces to the load board on which theDUT is placed. Bin box 310 is located near test head 309 and typicallycontains START and RESET buttons and displays pass/fail results.

Depending on its test purposes, it is to be appreciated that an ATE mayhave more or fewer than the components discussed above. Further, itshould be clear that the components of the ATE discussed above areconventional and well known by people of ordinary skill in the art.

The voltage-to-current converter (the current source) under the presentinvention is implemented as part of the PE circuitry inside test head309. In accordance to a first embodiment of the present invention, byimplementing a programmable digital-to-analog (D/A) converter betweenthe bases of the two transistors in a voltage controlled current sourcecircuit, a dynamic, programmable, and open-loop current source can beconstructed. Under this embodiment, the current source can be programmedby computer system 301 to send the desired current to the DUT.

Reference is now made to FIG. 4 illustrating the first embodiment of thecurrent source in accordance to the present invention. As shown in FIG.4, current source 400 consists of bipolar transistors 401-402, resistorR_(set) 403, resistor R_(ref) 404, and programmable D/A converter 405.Preferably, transistors 401 and 402 are located on the same substrate sothat their electrical and thermal characteristics are substantiallymatched. Also, resistor R_(set) 403 and resistor R_(ref) 404 aresubstantially equal. The emitter of transistor 401 receives a referencevoltage V_(ref) and is also connected to resistor R_(set) 403. The baseof transistor 401 is connected to resistor R_(ref) 404, the collector oftransistor 401, and the voltage reference input of D/A converter 405.D/A converter 405 receives a programmable value PV from computer system401. Its output is connected to the base of transistor 402 which is alsoconnected to resistor R_(ref) 404. The emitter of transistor 402 isconnected to resistor R_(set) 403. The collector of transistor 402provides the output I_(out) for current source 400.

In so doing, transistors 401-402, resistor R_(set) 403, and resistorR_(ref) 404 make up a voltage controlled current source whereintransistor 401 provides the temperature tracking voltage reference andtransistor 402 acts as a voltage-to-current converter. Further, itshould be clear to a person of ordinary skill in the art that with itsbase connected to its collector, transistor 401 acts like a diodedevice. Hence, a diode device that has similar characteristics mayreplace transistor 401. Programmable D/A converter 405 is placed betweenthe bases of transistors 401 and 402 to allow current source 400 to beprogrammable. As such, programmable D/A converter 405 acts as thevoltage reference circuit. The D/A converter's voltage reference inputand hence its output will now track transistor 402 ΔV_(BE).

A circuit analysis of current source 400 is performed to determine anequation for the output current I_(out). It is well known that thevoltage V_(out) of a programmable D/A converter is equal to:

    V.sub.out =V.sub.RefIn *(PV/FS)                            (12)

where PV is the digital programmable value from controlled systemcomputer 301 and FS is the digital full scale value of D/A converter405.

Following the logic of the analysis in deriving equation (4) in thebackground section, current I₁ is determined to be:

    I.sub.1 =(V.sub.Ref -V.sub.BE1 -V.sub.out)/R.sub.Ref       (13)

where V_(BE1) is the voltage between the base and emitter of transistor401. Similarly, current I₂ is:

    I.sub.2 =(V.sub.Ref -V.sub.BE2 -V.sub.out)/R.sub.set       (14)

Since V_(BE1) =V_(BE2) (transistors 401 and 402 are substantiallyequivalent) and R_(set) =R_(Ref), it can be shown that I₁ ≈I₂. FromKirchoff's current law:

    I.sub.2 =I.sub.out +I.sub.Base                             (15)

where I_(Base) is the base current of transistor 402.

Current I_(Base) is approximately equal to I₂ /H_(fe), where H_(fe) isthe transistor gain which is typically in the range of 150-300. Thus,I_(Base) is negligible and I₂ ≈I_(out) under equation (15). Substitutingequation (12) into equation (15):

    I.sub.2 ≈I.sub.out =(V.sub.Ref -V.sub.BE2 -(V.sub.Refin *(PV/FS)))/R.sub.set                                      (16)

By inspection, V_(Refin) =V_(Ref) -V_(BE1) =V_(Ref) -V_(BE2). When thisis substituted into equation (16), equation (16) becomes:

    I.sub.out =(V.sub.Ref -V.sub.BE2 -((V.sub.Ref -V.sub.BE2)*(PV/FS)))/R.sub.set                           (17)

Factoring out the term ((V_(Ref) -V_(BE2))/R_(set)) equation (17)becomes:

    I.sub.out =((V.sub.Ref -V.sub.BE2)/R.sub.set)*(1-(PV/FS))  (18)

From equation (17), it can be seen that when the ratio (PV/FS)approaches unity, the V_(BE) terms cancel out as I_(out) approacheszero. The significance of this is that small values of current can beprogrammed with negligible temperature affect. Referring now to FIG. 4Aillustrating, as an example, the I-V curves of the first embodiment inaccordance to the present invention. As shown in FIG. 4A, under thepresent invention, the error ΔV_(BE) @ΔT is proportional to the current.Hence, at very low current values, the error ΔV_(BE) @ΔT is essentiallyreduced to zero (0).

Moreover, under the present invention, there is no unusable range and nooffset changes because V_(Refin) tracks the V_(BE) voltage of transistor402. And, in accordance to the present invention, the whole voltageoperating range of the current source is available. This can beillustrated by the considering the same example discussed earlier in thebackground. As before, assume that V_(Ref) =5V, I₀ =1 mA, V_(BE) =0.6V,and that the D/A converter is a 12-bit converter. The resolution forthis 12-bit D/A converter is 5V/2¹² bit=1.22 mV/bit. Unlike the priorart example, the current I_(out) under equation (18) is maximum when PVis 0 and minimum when PV=FS. Hence, there is no unusable voltage rangeand no unusable D/A converter bit range in accordance to the presentinvention.

In accordance to a second embodiment of the present invention, byimplementing a variable differential amplifier between the bases of thetwo transistors in a current mirror circuit, a dynamic, variable, andopen-loop current source can be constructed. Under this embodiment, thecurrent source can be varied by changing the values of resistors R_(I)and R_(F) to program the desired current to the DUT.

Referring now made to FIG. 5 illustrating a second embodiment of thecurrent source in accordance to the present invention. As shown in FIG.5, current source 500 consists of bipolar transistors 501-502, resistorR_(I1) 503, resistor R_(F1) 504, resistor R_(I2) 505, resistor R_(F2)506, resistor R_(set) 507, and operational amplifier (op-amp) 508.Preferably, transistors 501 and 502 are located on the same substrate sothat their electrical and thermal characteristics are substantiallymatched. Also, resistor R_(I1) 503 and resistor R_(F1) 504 arepreferably equal to their counterparts resistor R_(I2) 505 and resistorR_(F2) 506. However, depending on needs, it is to be appreciated thatresistor R_(I1) 503 can have a different value than its counterpartresistor R_(I2) 505 and resistor R_(F1) 504 can have a different valuethan its counterpart resistor R_(F2) 506.

The emitter of transistor 501 receives a reference voltage V_(ref). Thebase of transistor 501 is connected to resistor R_(I1) 503 and thecollector of transistor 501. Resistor R_(I1) 503 is in turn connected tothe non-inverted input of op-amp 508. Resistor R_(F1) 504, which is inparallel to resistor R_(I1) 503, is also connected to the non-invertedinput of op-amp 508. The other end of resistor R_(F1) 504 is connectedground GND. The inverted input of op-amp 508 is connected to resistorR_(I2) 505 which in turn is connected to the voltage source V_(i). Theinverted input of op-amp 508 is also connected to R_(F2) 506 which inturn is connected to the output of op-amp 508. The output of op-amp 508is connected to resistor R_(set) 507 which in turn is connected to thebase of transistor 502. The emitter of transistor 502 receives voltageV_(Ref). The collector of transistor 502 provides the output currentI_(out) of current source 500.

In so doing, transistors 501 and 502 make up a voltage controlledcurrent source circuit wherein transistor 501 provides temperaturetracking voltage reference and transistor 502 is the voltage-to-currentconverter. Further, it should be clear to a person of ordinary skill inthe art that with its base connected to its collector, transistor 501acts like a diode device. Hence, a diode device with similarcharacteristics may replace transistor 501. On the other hand, op-amp508, resistor R_(I1) 503, resistor R_(F1) 504, resistor R_(I2) 505, andresistor R_(F2) 506 make up a differential amplifier which together withresistor Rset 507 are placed between the bases of transistors 501 and502 to act as a voltage reference circuit wherein the values ofresistors R_(Ii) and R_(Fi) is used to program the desired current tothe DUT. The voltage reference circuit's voltage reference input andhence its output will now track transistor 502 ΔV_(BE).

A circuit analysis of the differential amplifier indicates that:

    V.sub.out =(R.sub.F /R.sub.I)*(V.sub.Ref -V.sub.BE1)-V.sub.i)(19)

where R_(I1) 503=R_(I2) 505=R_(I) and R_(F1) 504=R_(F2) 506=R_(F) andV_(BE1) is the base-emitter voltage of transistor 501.

The output current I_(out) is equal to:

    I.sub.out =(V.sub.Ref -V.sub.BE2 -V.sub.out)/R.sub.set     (20)

where V_(BE2) is the base-emitter voltage of transistor 502.

By substituting equation (19) into equation (20), equation (20) thenbecomes:

    I.sub.out =(V.sub.Ref -V.sub.BE2 -((V.sub.Ref -V.sub.BE1)-V.sub.i)*(R.sub.F /R.sub.I))/R.sub.set                                      (21)

If R_(F) /R_(I) =1, then V_(BE2) =V_(BE1) and equation (21) is reducedto:

    I.sub.out =V.sub.i /R.sub.set                              (22)

According to equation (22), the output current I_(out) is not dependenton V_(BE) and therefore, is not subject to temperature variations.Therefore, the second embodiment of the present invention can operatewith small values of current since the temperature affect is negligible.Moreover, under the present invention, there is no unusable range and nooffset changes because V_(Refin) tracks the V_(BE) voltage of transistor502.

The two embodiments of the present invention, a current source (a.k.a.voltage-to-current converter) circuit, are thus described. While thepresent invention has been described in particular embodiments, thepresent invention should not be construed as limited by suchembodiments, but rather construed according to the below claims.

What is claimed is:
 1. A current source circuit comprising:a voltagecontrolled current source circuit comprising first and secondtransistors each having a base, a collector, and an emitter, theemitters of the first and second transistors being coupled to a firstvoltage, the collector and the base of the first transistor beingconnected together, the collector of the second transistor providing anoutput current for the current source circuit; and a voltage referencecircuit coupled between the bases of the first and second transistorsfor producing a control voltage at the base of said second transistor inresponse to a control signal appearing at the base of said firsttransistor, wherein a magnitude of said control voltage varies withtemperature of the first transistor, wherein the voltage referencecircuit comprises a programmable digital-to-analog (D/A) converter forreceiving input control data and said control signal and for producingsaid control voltage of magnitude determined by a combination ofmagnitudes of said control signal and said control data.
 2. The currentsource circuit of claim 1 further comprising:a first resistor coupledbetween the bases of the first and second transistors; and a secondresistor coupled between the emitters of the first and secondtransistors.
 3. The current source circuit of claim 2, wherein theoutput current has a magnitude I_(out) in accordance with the equation:

    I.sub.out =((V.sub.Ref -V.sub.BE2)/R.sub.set)*(1-(PV/FS))

where V_(Ref) is a magnitude of the first voltage, V_(BE2) is amagnitude of the base-emitter voltage of the second transistor, R_(set)is a magnitude of the second resistor, PV is a current valve of thecontrol data, and FS is a maximum valve of the control data.
 4. Acurrent source circuit comprising:a voltage controlled current sourcecircuit comprising first and second transistors each having a base, acollector, and an emitter, the emitters of the first and secondtransistors being coupled to a first voltage, the collector and the baseof the first transistor being connected together, the collector of thesecond transistor providing an output current for the current sourcecircuit; a voltage reference circuit coupled between the bases of thefirst and second transistors for producing a control voltage at the baseof said second transistor in response to a control signal appearing atthe base of said first transistor, wherein a magnitude of said controlvoltage varies with temperature of the first transistor, wherein thevoltage reference circuit comprises a differential amplifier amplifyingsaid control signal to produce said control voltage, and a firstresistor coupled between the differential amplifier and the base of thesecond transistor, wherein the differential amplifier comprises:anoperational amplifier (op-amp) having an inverted input, a non-invertedinput, and an output; a second resistor connected to the non-invertedinput of the op-amp; a third resistor coupled between the non-invertedinput of the op-amp and a second voltage; a fourth resistor coupledbetween the inverted input of the op-amp and a voltage source; and afifth resistor coupled between the inverted input and the output of theop-amp.
 5. The current source of claim 4, wherein the second and fourthresistors have substantially equal resistance and the third and fifthresistors have substantially equal resistance.
 6. The current source ofclaim 5, wherein the output current has a magnitude I_(out) inaccordance with the equation:

    I.sub.out =(V.sub.Ref -V.sub.BE2 -((V.sub.Ref -V.sub.BE1)-V.sub.i)*(R.sub.F /R.sub.I))/R.sub.set

where V_(Ref) is the first voltage, V_(BE1) is the base-emitter voltageof the first transistor, V_(BE2) is a base-emitter voltage of the secondtransistor, V_(i) is a magnitude of the second voltage, R_(F) is aresistance of the fourth resistor, R_(I) is a resistance of the secondresistor, and R_(set) is a resistance of the first resistor.
 7. AnAutomatic Test Equipment (ATE) comprising:a test head comprising a pinelectronics (PE) circuit for supplying input signals to a device undertest (DUT) and for receiving output signals from the DUT, the PE circuitcomprising a current source circuit comprising:a voltage controlledcurrent source circuit comprising first and second transistors eachhaving a base, a collector, and an emitter, the emitters of the firstand second transistors being coupled to a first voltage, the collectorand the base of the first transistor being connected together, thecollector of the second transistor providing an output current for thecurrent source circuit; and a voltage reference circuit coupled betweenthe bases of the first and second transistors for producing a controlvoltage at the base of said second transistor in response to a controlsignal appearing at the base of said first transistor, wherein amagnitude of said control voltage varies with temperature of the firsttransistor, wherein the voltage reference circuit comprises aprogrammable digital-to-analog (D/A) converter for receiving inputcontrol data and said control signal and for producing said controlvoltage of magnitude determined by a combination of magnitudes of saidcontrol signal and said control data.
 8. The ATE of claim 7, wherein thecurrent source further comprising:a first resistor coupled between thebases of the first and second transistors; and a second resistor coupledbetween the emitters of the first and second transistors.
 9. The ATEsource circuit of claim 8, wherein the output current has a magnitudeI_(out) in accordance with the equation:

    I.sub.out =((V.sub.Ref -V.sub.BE2)/R.sub.set)*(1-(PV/FS))

where V_(Ref) is a magnitude of the first voltage, V_(BE2) is amagnitude of the base-emitter voltage of the second transistor, R_(set)is a magnitude of the second resistor, PV is a current value of thecontrol data, and FS is a maximum value of the control data.
 10. AnAutomatic Test Equipment (ATE) comprising a test head comprising a pinelectronics (PE) circuit for supplying input signals to a device undertest (DUT) and for receiving output signals from the DUT, the PE circuitcomprising a current source circuit comprising:a voltage controlledcurrent source circuit comprising first and second transistors eachhaving a base, a collector, and an emitter, the emitters of the firstand second transistors being coupled to a first voltage, the collectorand the base of the first transistor being connected together, thecollector of the second transistor providing an output current for thecurrent source circuit; a voltage reference circuit coupled between thebases of the first and second transistors for producing a controlvoltage at the base of said second transistor in response to a controlsignal appearing at the base of said first transistor, wherein amagnitude of said control voltage varies with temperature of the firsttransistor, wherein the voltage reference circuit comprises adifferential amplifier amplifying said control signal to produce saidcontrol voltage, and a first resistor coupled between the differentialamplifier and the base of the second transistor, wherein thedifferential amplifier comprises:an operational amplifier (op-amp)having an inverted input, a non-inverted input, and an output; a secondresistor connected to the non-inverted input of the op-amp; a thirdresistor coupled between the non-inverted input of the op-amp and asecond voltage; a fourth resistor coupled between the inverted input ofthe op-amp and a voltage source; and a fifth resistor coupled betweenthe inverted input and the output of the op-amp.
 11. The ATE of claim10, wherein the second and fourth resistors are of substantially equalresistance and the third and fifth resistors are of substantially equalresistance.
 12. The ATE of claim 11, wherein the output current has amagnitude I_(out) in accordance with the equations:

    I.sub.out =(V.sub.Ref -V.sub.BE2 -((V.sub.Ref -V.sub.BE1)-V.sub.i)*(R.sub.F /R.sub.I))/R.sub.set

where V_(Ref) is a magnitude of the first voltage, V_(BE2) is amagnitude of the base-emitter voltage of the second transistor, R_(set)is a magnitude of the second resistor, PV is a current valve of thecontrol data, and FS is a maximum valve of the control data.
 13. Amethod for converting voltage into an output current comprising thesteps of:in a voltage controlled current source circuit comprising firstand second transistors each having a base, a collector, and an emitter,the emitters of the first and second transistors being coupled to afirst voltage, the collector and the base of the first transistorconnected together, providing said output current at the collector ofthe second transistor; and producing a control voltage at the base ofsaid second transistor in response to a control signal appearing at thebase of said first transistor, wherein a magnitude of said controlvoltage varies with temperature of the first transistor, wherein thestep of producing said control voltage comprises the substepsof:connecting an input of a programmable digital-to-analog (D/A)converter to said base of said first transistor, and connecting anoutput of said D/A converter to the base of said second transistor. 14.A current source circuit comprising:a voltage controlled current sourcecomprising a diode having an input and an output and a first transistorhaving a base, a collector, and an emitter, the input of the diode andthe emitter of the first transistor being coupled to a first voltage,the collector of the first transistor providing an output current forthe current source circuit; and a voltage reference circuit coupledbetween the output of the diode and the base of the first transistor forproducing a control voltage at the base of said first transistor inresponse to a control signal appearing at the output of said diode,wherein a magnitude of said control voltage varies with a temperature ofthe diode, wherein the voltage reference circuit comprises aprogrammable digital-to-analog (D/A) converter.
 15. An apparatus forgenerating an output current of magnitude controlled by input data,comprising:a first voltage source:a first transistor having a firstbase, a first collector and a first emitter, the first emitter beingcoupled to said first voltage source, said output current beinggenerated in the first collector; a digital-to-analog (D/A) converter,having a first input receiving a reference signal and having a secondinput receiving said input data, for producing a control voltage ofmagnitude determined by a combination of magnitudes of said referencesignal and said input data, said control voltage being applied to saidfirst base; and a diode providing a conductive path between said firstvoltage source and said first input of said D/A converter therebysupplying said reference signal to said DAC, wherein the magnitude ofsaid control voltage varies with temperature of said diode.
 16. Thecurrent source in accordance with claim 15 wherein said diode is formedby a second transistor having a second base, a second collector and asecond emitter, the second emitter being coupled to said first voltagesource and the second base and second collector being coupled to saidfirst input of said D/A converter.
 17. The current source in accordancewith claim 16 wherein said first transistors and said second transistorare substantially similar.
 18. The current source in accordance withclaim 17 further comprising:a first resistor coupling said secondemitter to said first voltage source, and a second resistor couplingsaid second base to said first base, wherein said first and secondresistors are of substantially similar resistance.
 19. A current sourcefor generating an output current comprising:a first voltage source: afirst transistor having a first base, a first collector and a firstemitter, the first emitter being coupled to said first voltage source,said output current being generated in the first collector; an amplifiercircuit having an amplifier input for receiving a reference current andfor producing a control voltage at said first base of said firsttransistor in response to said reference current; and a diode forconnected for conducting said reference current between first voltagesource and said amplifier input, wherein said diode is formed by asecond transistor having a second base, a second collector and a secondemitter, the second emitter being connected to said first voltage sourceand said second base and said second collector being connected to saidamplifier input, wherein said first transistor and said secondtransistor have substantially similar electrical and thermalcharacteristics, and wherein said amplifier circuit comprises:a circuitground; an operational amplifier having a first operational amplifierinput, a second operational amplifier input, and an operationalamplifier output; a first resistor connecting said second collector andsaid second base to said first operational amplifier input; a secondresistor coupling said first operational amplifier input said circuitground; a second voltage source; a third resistor coupling said secondoperational amplifier input to said second voltage source; a fourthresistor coupling said operational amplifier output to said first base;and a fifth resistor coupling said operational amplifier output to saidsecond operational amplifier input.